Copy rollershutter and garage_door docs to cover

Re: home-assistant/home-assistant#1949

source/_components/cover.command_line.markdown

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+---
+layout: page
+title: "Command Line Cover"
+description: "How to control a cover with the command line."
+date: 2016-06-28 17:30
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: command_line.png
+ha_category: Cover
+---
+A cover platform that issues specific commands when it is moved up, down and
+stopped. It allows anyone to integrate any type of cover into Home Assistant
+that can be controlled from the command line.
+
+To enable a command line cover in your installation, add the following to your
+`configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+cover:
+- platform: command_line
+  covers:
+    Garage door:
+      open_cmd: move_command up kitchen
+      close_cmd: move_command down kitchen
+      stop_cmd: move_command stop kitchen
+      state_cmd: state_command kitchen
+      value_template: {% raw %}>
+        {% if value == 'open' %}
+        100
+        {% elif value == 'closed' %}
+        0
+        {% endif %}
+        {% endraw %}
+```
+
+Configuration variables:
+
+- **covers** (*Required*): The array that contains all command line covers.
+  - **entry** (*Required*): Name of the command line cover. Multiple entries
+    are possible.
+    - **open_cmd** (*Required*): The command to open the cover.
+    - **close_cmd** (*Required*): The action to close the cover.
+    - **stop_cmd** (*Required*): The action to stop the cover.
+    - **state_cmd** (*Optional*): If given, this will act as a sensor that runs
+      in the background and updates the state of the cover. If the command
+      returns a `0` the indicates the cover is fully closed, whereas a 100
+      indicates the cover is fully open.
+    - **value_template** (*optional - default: '{% raw %}{{ value }}{% endraw
+      %}'*): if specified, `state_cmd` will ignore the result code of the
+      command but the template evaluating will indicate the position of the
+      cover. For example, if your `state_cmd` returns a string "open",
+      using `value_template` as in the example config above will allow you to
+      translate that into the valid state `100`.